1. Field of the Invention
This invention relates to ultrasonic testing and more particularly relates to non-destructive ultrasonic testing technique for the measurement of thickness of a test piece which includes determination of the velocity of ultrasonic energy in the test piece.
2. Discussion of the Relevant Art
There are many ultrasonic instruments which have been used for measuring the thickness from one side of the material with the pulse-echo method. Such instruments require a calibration technique using test blocks of similar material or by knowing the actual acoustic velocity of the material being measured, and then calibrating the instrument correspondingly.
One method requires the use of two or more test samples having the same or similar acoustic velocity as the material to be measured. These test samples have known thicknesses and must cover the range of measurement expected. The readout of the thickness measuring device is calibrated by first setting the zero control on the thinner of the two test pieces, and then setting the span control on the thicker of the two test pieces.
Another calibration technique uses the fact that the ultrasonic thickness measuring instrument can be calibrated in terms of acoustic velocity. If the acoustic velocity of the material being measured is known, this information can be set into the instrument and only one test piece is required to achieve a zero offset.
Material or structure such as pressure containment vessels without two exposed and opposed measurable surfaces are not presently measurable for velocity. The acoustic velocity is a function of the square root of the ratio of the bulk elastic modulus of the test material to its density. In some materials, particularly cast iron, the acoustic velocity may vary from sample to sample plus or minus 15% of a nominal value which is generally considered to be 2.0.times.10.sup.5 inches per second. If the actual acoustic velocity of the material under test is not known, thickness testing by the ultrasonic pulse echo method using a nominal value of acoustic velocity may be quite erroneous.
Accordingly, the present invention provides a new and improved technique for determining the actual acoustic velocity of material to be tested where access to the material may be had from only one side thereof and thereafter measuring the thickness of the material.
Ultrasonic test equipment of the pulse-echo type generally may be considered to make thickness measurements using the velocity factor of the material as a base reference. Generally, in such equipment, the time is measured between a transmitted pulse and the received echo. This may be achieved by generating a rectangular waveform whose duration is a measure of the transit time of the acoustic wave between opposed surfaces. Knowing the transit time and the acoustic velocity in the test material then leads to a simple arithmatic determination of the thickness of the material. Most ultrasonic thickness gauging instruments operate on this theory. Typical instruments are the DIGI-SONIC 502 manufactured by Sonic Instruments, Inc. of Trenton, N.J. and also a Model 220, manufactured by the same company. These instruments are portable, designed for field testing, and at least one will be partially described in conjunction with the invention.